Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

Editorial Board (March 2022)
Editor-in-Chief
Takeshi Sagiya, Nagoya University, Japan

Vice Editors-in-Chief
Masahito Nosé, Nagoya University, Japan

Aitaro Kato, University of Tokyo, Japan

Editors
Koki Aizawa
Hideo Aochi
Pascal Audet
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Xiaowei ChenPeter Chi
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Guangyu Fu
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Stuart Henrys
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Hauke Hussmann

Mie Ichihara
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Earth, Planets and Space (EPS) is the official journal of Society of Geomagnetism and Earth, Planetary
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Earth, Planets and Space(2021) 73:32
Matsuzawa et al. Earth, Planets and Space
https://doi.org/10.1186/s40623-020-01337-5

Open Access

PREFACESpecial issue "Crustal dynamics:
toward integrated view of island arc
seismogenesis"
Toru Matsuzawa1*, Ichiko Shimizu2, Takuya Nishimura3, Christopher J. Spiers4, Junichi Nakajima5
and Tatsuhiko Kawamoto6

earthquake, the large slip near the trench during the giant
rupture, and essential features of its postseismic deformation. Shi et al. (2020) modeled a representative crosssection of the Nankai subduction zone offshore Shikoku
Island, where the frictional behavior is dictated by the
structure and composition of the overriding plate. They
showed that mechanical interactions between neighboring fault segments and the impact from the long-term
viscoelastic flow strongly modulate the recurrence pattern of earthquakes and slow-slip events. Miyake andNoda (2019) investigated the effect of viscoelasticity on
the transition between seismogenic and aseismic behavior using a numerical simulation. Their results indicatethat the transition associated with slow-slip events andtremors is dominated by changes in frictional properties
rather than changes in viscoelastic properties.
Not only the large-scale heterogeneity mentioned
above, but local to regional scale inhomogeneities will
also affect the seismogenesis. Hisakawa et al. (2020)
investigated the rupture process of the 2018 Hokkaido
Eastern Iburi earthquake, Japan, by using dynamic rupture simulations to show that the observed complexityof the event can be explained primarily by the effect of
non-planar fault geometry with multiple bends. Tsudaet al. (2019) estimated the velocity structure in the Sanin district, southwestern Japan, using seismic travel-time
tomography to find that the lower crust beneath the seismic belt showed lower velocities. They concluded thatthe seismic belt is generated because the lower crust
beneath the belt is weak due to the high temperature in
the eastern part and the water dehydrated from the Philippine Sea plate in the western part. Suemoto et al. (2020)

Recent deployments of dense seismic and geodetic observation networks have revealed a detailed pattern of crustal stress and strain rate in tectonically active regions allover the world. Furthermore, the 2011 M9.0 Tohoku-oki
earthquake provided a unique opportunity to investigate
how the Japanese Islands’ crust responds to instantaneous as well as transient stress changes due to the giant
fault motion. This special issue includes 28 papers, which
are the results of state-of-the-art researches carried out
to understand the crustal dynamics and earthquake
generation process in the island arc system. The papers
might be categorized into several groups: (1) inhomogeneous structures related to earthquakes; (2) inelastic
deformation; (3) stress and fault strength; (4) fluids andearthquakes; and (5) characteristic of fluids.
It is evident that inhomogeneous structures affect seismic activities, but the detail of the effect is still ambiguous. Baba and Yoshida (2020) clarified the detailed
geological structure in the northeastern Japan forearc
and demonstrated a close relationship between seismic activity and the geological structure of the overriding plate. Barbot (2020) constructed a two-dimensionalrheological model for the northeastern Japan subduction zone. He showed that most of the partial rupturesof the megathrust are structurally controlled, and themodel consistently explains the characteristics of historical earthquakes in the Miyagi segment, the slow-slip
and foreshock preparatory phase of the 2011 Tohoku-oki*Correspondence: toru.matsuzawa.c6@tohoku.ac.jp
1
Graduate School of Science, Tohoku University, Sendai 980‑8578, Japan
Full list of author information is available at the end of the article© The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) andthe source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material
in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material
is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds thepermitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativeco
mmons.org/licenses/by/4.0/.

1Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis
Matsuzawa et al. Earth, Planets and Space(2021) 73:32

Page 2 of 4

the 1943 Tottori earthquake, while the inelastic zones arelocated just below the source regions of the 2000 Western and the 2016 Central Tottori earthquakes.
When an earthquake occurs, the shear stress on thefault must be larger than the strength. Thus, investigations of the stress concentration and strength
reduction are essential to understand the earthquake
generation process. Iio et al. (2020) investigated the temporal changes in the spatial distributions of hypocentersand focal mechanisms of aftershocks of the 2016 Central Tottori Prefecture earthquake, southwestern Japan,
using the data from a dense temporary seismograph network. They concluded that the aftershock activity wascontrolled mainly by stress concentration rather thanstrength reduction due to high fluid pressure. Mitogawa
and Nishimura (2020) calculated the temporal evolution of the Coulomb failure stress changes on the crustal faults in southwest Japan by utilizing viscoelastic
modeling of megathrust earthquake cycles. They foundthat the large inland earthquakes before the megathrust earthquakes can be explained only when the apparent frictional coefficient is less than ~ 0.1. Kameda et al.
(2019) demonstrated that the swelling pressure systematically increases with a decrease in sample porosity
from experiments using the fault material, composed
primarily of smectite, obtained by the IODP Expedition
343 JFAST program. The experiments suggest that themodified effective confining pressure of the fault is quite
low or potentially zero, which means the fault may be
intrinsically weak. In order to investigate the stress in theseismogenic zones, we need a great many focal mechanisms. Hara et al. (2019) developed a model of the convolutional neural networks (CNNs) to determine P-wave
first-motion polarities of observed seismic waveforms
automatically. They found that the accuracies of the CNN
models were generally high (≳95%) and that regional
dependence was insignificant.
Among all the possible causes of strength reduction,
an increase in the pore fluid pressure is the most probable candidate. In and around many fault zones, Sibson(2020) found many geological and geophysical characteristics that support his "fault-valve" model, especiallyin compressional–transpressional tectonic regimes,
which are better at containing overpressure and are‘load-strengthening’ (mean stress rising with increasing
shear stress). Maeda et al. (2019) investigated the focal
mechanisms of aftershocks of the 2008 Iwate–Miyagi
Nairiku earthquake (M 7.2) in the northeastern Japan arc
and detailed Vp/Vs structure there. They concluded thatexceptional N–S compressional aftershocks occurredin regions with low strength due to the high pore pressure and with N–S compressional stress caused by thestress change due to the mainshock in a low differential

derived a three-dimensional S-wave velocity modelfor the San-in area of southwest Japan using ambient
noise data from a dense seismic network. In the resulting model, faults and a previously unrecognized tectonic
boundary appeared as low-velocity anomalies or velocity
boundaries, and the velocity anomalies were also associated with many past earthquake hypocenters.
Since earthquakes release the elastic strain energy accumulated in the plates, elastic strain estimation is crucial
to assess large earthquakes’ potential. However, inelastic
deformation is also essential to understand the topography and stress concentration, both of which are strongly
related to earthquakes. Fukuda et al. (2020) investigatedcooling/denudation history in the northern part of thenortheastern Japan arc using thermochronometry. The
results indicate slow denudation rates in the fore-arc
side and high rates in the Ou Backbone Range and on
the back-arc side, similar to the southern part of the arc.
Fukahata et al. (2020) successfully separated plastic/viscous deformation from the observed strain in the northern Niigata–Kobe Tectonic Zone (NKTZ), central Japan,
using GNSS data before and after the 2011 Tohoku-oki
earthquake. Their results indicate that the strain rate was
exceptionally faster before the Tohoku-oki earthquake
because of plastic strain, and the discrepancy between
the geodetic and geologic strain rates is much smaller inmost of the period. Otsubo et al. (2020) investigated therelationship between contractional deformation of sedimentary mass in the upper crust and the geodetic strain
rate in a high-strain zone of the mid-Niigata region, central Japan, to find numerous examples of layer-parallel
slip (bedding-plane slip) generated by folding. The results
support an existing model, based on geodetic observations, of mechanical decoupling between the weak
sedimentary layers and basement. Dojo and Hiramatsu(2019) analyzed the temporal variation in coda Q in thenortheastern part of the NKTZ in central Japan. They
found no statistically significant temporal variations inthe spatial distribution of coda Q in the periods before
and after the 2011 Tohoku-oki earthquake, which implies
the existence of a persistent ductile deformation in theupper crust contributes essentially to the generation process of the high strain rate in the northeastern part of the
NKTZ. Tamura et al. (2020) quantified the slip rate attributable to minor faults in the southeastern-central part of
the NKTZ to the south of the Atotsugawa Fault, central
Japan. Their results indicate that these minor faults contribute 4–24% of the total crustal strain in NKTZ. Meneses-Gutierrez and Nishimura (2020) analyzed GNSS data
within the San-in Shear Zone, southwestern Japan, toclarify the width of the inelastic deformation zone in thelower crust beneath it. In eastern Tottori, the estimatedinelastic zone’s center coincides with the source region of2

Earth, Planets and SpaceMatsuzawa et al. Earth, Planets and Space(2021) 73:32

Page 3 of 4

reflect grain-boundary water. Kuwatani and Toriumi(2020) developed a kinetic model that illustrates the relative roles of intergranular diffusion and surface reactions
in overall metamorphic net-transfer reactions involving
solid solutions. Their conceptual model can be extended
to capture more complex behaviors of metamorphic
reactions in natural systems. Watanabe et al. (2019) conducted elastic wave velocity and electrical conductivity
measurements in a brine-saturated granitic rock sample
in the laboratory. Electron microscopic observation ofpores in the sample showed remarkable variation in theaperture of each crack. At high pressures, narrow aperture parts are closed while wide aperture parts are still
open to maintain conduction paths, explaining the variations in the seismic wave velocity and electrical conductivity observed in the field.

stress regime. Matsumoto et al. (2020) investigated inelastic strain decay rates using aftershock moment tensor
data obtained from dense seismic observations in the
focal area of the 2000 M7.3 Western Tottori earthquake,southwestern Japan. They found an increase in the spatial variations in the inelastic strain rate, and they concluded that the decay in the inelastic strain rate is mainly
controlled by a power-law fluid and slow decay in someregions might be attributed to aseismic slip. Amezawa
et al. (2019) found distinct scattered wave packets (DSW)in the waveforms of S-coda from earthquakes around theMoriyoshi-zan volcano in the northeastern Japan arc.
Since the DSW origin is located between the large clusterof earthquakes and a low-velocity zone, they concluded
that the DSW origin was composed of geofluid accumulated midway in the upward fluid movement from thelow-velocity zone to the earthquake cluster. Hutapea
et al. (2020) developed a continuous monitoring system of the seismic velocity of the Japanese Islands using
seismic interferometry. The system clearly shows spatiotemporal seismic velocity changes that could be related
to pore pressure and/or magma variations.
As mentioned above, fluids can be the most responsible for reducing the fault strength. In order to model thestrength reduction, however, we have to understand thedetailed characteristics of fluids. Kusuhara et al. (2020)
showed that the isotopic characteristics of brine samples
from the Kashio mineral spring, central Japan, indicatedthat the Kashio brine could have originated from fluids
dehydrated from the Philippine Sea slab as well as the
Arima hot spring, located at 250 km west of the Kashio.
They evaluated the chemical difference between those
two springs with the depths of the subducting Philippine
Sea slab and also possible interaction with crustal rocks
above it. Koizumi et al. (2019) analyzed daily streamflow
data collected in regions with strong ground motion during the 2016 Kumamoto earthquake. They concluded
that the postseismic hydrological changes were caused
mainly by earthquake-induced surface phenomena andlittle contribution from a hydrothermal fluid. Sato et al.
(2020) investigated long-term hot spring discharge triggered by the 2011 Mw 6.6 Iwaki earthquake, northeasternJapan, and they concluded that such long-term discharge
might be explained by the rise of thermal water from thedeep part and the permeability changes along the hotspring channels. Fukuda and Shimizu (2019) investigatedvariation in water contents in quartz in the Sanbagawa
Metamorphic Belt, Japan. Using an infrared spectroscopic mapping technique, they measured the quartz’s
water contents to be 40–310 ppm. The lowest values
were obtained for large quartz grains in the high-grade
metamorphic zones. They speculated that relatively highwater contents in the lowest metamorphic grade zone

Abbreviations
CNN: Convolutional Neural Networks; DSW: Distinct scattered wave packets;
GNSS: Global Navigation Satellite System; IODP: Integrated Ocean Drilling Pro‑
gram; JFAST: Japan trench FAST drilling project; NKTZ: Niigata–Kobe Tectonic
Zone.
Acknowledgements
We express our sincere gratitude to the authors who contributed to thisspecial issue and the reviewers who evaluated the contributions and gavethoughtful comments and suggestions. We also thank Editor‑in‑Chief Yasuo
Ogawa and Vice Editor‑in‑Chief Masato Furuya for waiting patiently for ourslow editing process.
Authors’ contributions
All authors of this article are guest editors for this special issue. All authors readand approved the final manuscript.
Funding
This study was partly supported by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of Japan, under the Earthquake and Volcano
Hazards Observation and Research Program, and KAKENHI Grant Numbers
26109001–26109007, and 15K21755.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Graduate School of Science, Tohoku University, Sendai 980‑8578, Japan.
2
Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606‑8502,
Japan. 3 Disaster Prevention Research Institute, Kyoto University, Uji 611‑0011,
Japan. 4 Department of Earth Sciences, Faculty of Geosciences, Utrecht Uni‑
versity, Utrecht 3584 CB, the Netherlands. 5 Department of Earth and Planetary
Sciences, School of Science, Tokyo Institute of Technology, Tokyo 152‑8551,
Japan. 6 Department of Geoscience, Faculty of Science, Shizuoka University,
Shizuoka 422‑8529, Japan.
Received: 8 December 2020 Accepted: 9 December 2020

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Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in pub‑lished maps and institutional affiliations.

4Earth, Planets and SpaceFRONTIER LETTER

Open Access

Frictional and structural controls of seismic super-cycles at
the Japan trench
Sylvain Barbot
Earth, Planets and Space 2020, 72:63 DOI: 10.1186/s40623-020-01185-3
Received: 2 November 2019, Accepted: 24 April 2020, Published: 11 May 2020

Abstract
The diverse mechanical behaviors of subduction zones during the seismic cycle emerge from the nonlinear dynamics of a complex
mechanical system with interacting brittle and ductile deformation. The 2011 Tohoku mega-quake represents the culmination of asuper-cycle of partial and full ruptures of the plate interface, but the physical controls on the down-dip segmentation of the megathrustremain unclear. Here, we propose a two-dimensional rheological model of the Japan trench to explain the variability of earthquake sizeat the Miyagi section, in northern Honshu, during the last century. We simulate seismicity in a continuum with a physics-based rate- andstate-dependent constitutive law for fault slip, producing aperiodic earthquake sequences with a power-law distribution of rupture sizes.
Although some partial ruptures of the megathrust are the result of self-emergent behavior, others are structurally controlled. The 1978
and 2005 Mj ~ 7 interplate earthquakes took place in a metamorphic belt in the mantle wedge corner bounded up-dip by the arc Mohothat constitutes a permanent structural boundary. We explain the succession of thegreat 1981 Mw = 7.1 and 2003 Mw = 6.9 earthquakes within the forearc and the 2011
giant earthquake as the natural response of a large continuously velocity-weakening
fault with a small nucleation size. The shallow segment below the frontal prism only
slips in giant through-going ruptures that unzip the whole velocity-weakening
interface. The model consistently explains the size, recurrence time, and hypocenter
location of historical earthquakes in the Miyagi segment, the slow-slip and foreshock
preparatory phase of the 2011 Tohoku earthquake, the large slip near the trench
during the giant rupture, and important features of its postseismic deformation. The
complex patterns of seismicity at the Japan trench can be better understood byassimilating geological and geophysical observations at various periods of theseismic cycle within an explicative physical framework.

Keywords: Friction, Rheology, Subduction Zone, Seismic cycle, Japan trench

Graphical abstract

Corresponding author: Sylvain Barbot, sbarbot@usc.edu

FULL PAPER

Open Access

Temporal stability of coda Q in the northeastern part of aninland high strain rate zone, central Japan: implication of apersistent ductile deformation in the crust
Masanobu Dojo and Yoshihiro Hiramatsu*
Earth, Planets and Space 2019, 71:32 DOI: 10.1186/s40623-019-1013-y
Received: 17 December 2018, Accepted: 9 March 2019, Published: 15 March 2019

Abstract
We have analyzed the temporal variation in coda Q in the northeastern part of an inland high strain rate zone, the Niigata–Kobe Tectonic
Zone (NKTZ), in central Japan, to investigate the response of coda Q to the modification of the strain field induced by the 2011 Tohoku
earthquake (Mw 9.0). We observe no statistically significant temporal variations in the spatial distribution of coda Q as the wholeanalyzed area, implying that the crustal deformation induced by the 2011 Tohoku earthquake has provided no significant temporalvariation in crustal heterogeneity as the whole analyzed area. For the middle frequency bands, before and after the 2011 Tohoku
earthquake, we have commonly found a negative correlation between the spatial distributions of coda Q and the differential strain rate
and a positive correlation between the spatial distributions of coda Q and the perturbation of S-wave velocity in the upper crust. Thesefeatures, together with previous works, suggest that the ductile deformation with a high rate in the upper crust plays an important rolein generating the high strain rate in the analyzed area not only before but also after the 2011 Tohoku earthquake. In other words, theexistence of a persistent ductile deformation in the upper crust
contributes essentially to the generation process of the highstrain rate in the northeastern part of the NKTZ. It is important
to note that the location of the persistent ductile deformationin the northeastern part of the NKTZ, mainly in the upper crust,differs from that in the central part of the NKTZ, mainly in thelower crust.

Keywords: Niigata–Kobe Tectonic Zone, Heterogeneity,
Aseismic deformation, The 2011 Tohoku
earthquake

Graphical abstract*Corresponding author: Yoshihiro Hiramatsu, yoshizo@staf.kanazawa-u.ac.jp
5Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

FULL PAPER

Open AccessCauses of the N–S compressional aftershocks of the E–W
compressional 2008 Iwate–Miyagi Nairiku earthquake(M7.2) in the northeastern Japan arc
Sumire Maeda*, Toru Matsuzawa, Keisuke Yoshida, Tomomi Okada and Takeyoshi Yoshida
Earth, Planets and Space 2019, 71:94 DOI: 10.1186/s40623-019-1073-z
Received: 10 May 2019, Accepted: 21 August 2019, Published: 3 September 2019

Abstract
In this study, we investigated the influence of local structural heterogeneities on aftershocks of the 2008 Iwate–Miyagi Nairiku
earthquake (M 7.2) that occurred in the northeastern Japan arc. Although this area is characterized by an E–W compressional
thrust faulting stress regime, many N–S compressional thrust-type and strike-slip-type earthquakes were also observed in theaftershock activity of the 2008 event. The occurrence of such N–S compressional
aftershocks indicates that the differential stress was very low before the main shock. We
investigated Vp/Vs structure in the aftershock area in detail to find most of the area showed
Vp/Vs as low as less than 1.70, which is consistent with the previous studies. Our resultshows, however, the aftershock area was dotted with high Vp/Vs small areas, whichsuggests the locations of abundant fluid-filled cracks. The comparison of the distributionsof Vp/Vs, the stress changes due to the mainshock, and focal mechanisms of theaftershocks indicate that the N–S compressional aftershocks are concentrated in regionswith high Vp/Vs and large N–S compressional stress change. From these results, weconcluded that the N–S compressional aftershocks occurred in regions with low strength
due to the high pore pressure and with N–S compressional stress caused by the stresschange due to the mainshock in a low differential stress regime.

Keywords: Microearthquakes, Stress field, Heterogeneous structure, Fluid, Geologicalstructure

Graphical abstract*Corresponding author: Sumire Maeda, maeda.sumire@aist.go.jp

FULL PAPER

Open AccessP-wave first-motion polarity determination of waveform
data in western Japan using deep learning
Shota Hara*, Yukitoshi Fukahata and Yoshihisa Iio
Earth, Planets and Space 2019, 71:127 DOI: 10.1186/s40623-019-1111-x
Received: 23 May 2019, Accepted: 18 November 2019, Published: 29 November 2019

AbstractP-wave first-motion polarity is the most useful information in determining the focal mechanisms of earthquakes, particularly for smaller
earthquakes. Algorithms have been developed to automatically determine P-wave first-motion polarity, but the performance level of theconventional algorithms remains lower than that of human experts. In this study, we develop a model of the convolutional neuralnetworks (CNNs) to determine the P-wave first-motion polarity of observed seismic waveforms under the condition that P-wave arrival
times determined by human experts are known in advance. In training and testing the CNN model, we use about 130 thousand 250 Hz
and about 40 thousand 100 Hz waveform data observed in the San-in and the northern Kinki regions, western Japan, where three to fourtimes larger number of waveform data were obtained in the former region than in the latter. First, we train the CNN models using 250 Hz
and 100 Hz waveform data, respectively, from both regions. The accuracies of the CNN models are 97.9% for the 250 Hz data and 95.4%
for the 100 Hz data. Next, to examine the regional dependence, we divide the waveform data sets according to the observation region,and then we train new CNN models with the data from one region and test them using the data from the other region. We find that theaccuracy is generally high (≳ 95%) and the regional dependence is within about 2%. This suggests that there is almost no need to retrain
the CNN model by regions. We also find that the accuracy issignificantly lower when the number of training data is lessthan 10 thousand, and that the performance of the CNN
models is a few percentage points higher when using 250
Hz data compared to 100 Hz data. Distribution maps, on
which polarities determined by human experts and the
CNN models are plotted, suggest that the performance of
the CNN models is better than that of human experts.

Keywords: Machine learning, Convolutional neural

Graphical abstract

network, P-wave first-motion polarity*Corresponding author: Shota Hara, hara.shota.67m@st.kyoto-u.ac.jp
6Earth, Planets and Space

FULL PAPER

Open AccessHydrological changes after the 2016 Kumamoto earthquake,Japan
Naoji Koizumi*, Shinsuke Minote, Tatsuya Tanaka, Azumi Mori, Takumi Ajiki, Tsutomu Sato,
Hiroshi A. Takahashi and Norio Matsumoto
Earth, Planets and Space 2019, 71:128 DOI: 10.1186/s40623-019-1110-y
Received: 2 August 2019, Accepted: 16 November 2019, Published: 29 November 2019

Abstract
The 2016 Kumamoto earthquake, whose main shock was an M7.3 event on April 16, 2016, 28 h after a foreshock of M6.5, caused
severe damage in and around Kumamoto Prefecture, Japan. It also caused postseismic hydrological changes in Kumamoto
Prefecture. In this study, we analyzed daily streamflow data collected by eight observation stations from 2001 to 2017 in regionsthat experienced strong ground motion during the 2016 Kumamoto earthquake. We
also surveyed 11 water springs in the region several times after the main shock.
Streamflow had no or slight change immediately after the earthquake; however, large
increases were recorded at some of the eight stations following a heavy rainfall that
occurred 2 months after the earthquake. A decrease in the water-holding capacity of
the catchment caused by earthquake-induced landslides can explain this delayed
streamflow increase. Conversely, earthquake-related changes to the spring flow rate
were not so clear. Water temperature and chemical composition of spring waters werealso hardly changed. Only the concentration of NO3−, which is usually considered to besupplied from the surface, changed slightly just after the earthquake. These results
show that the postseismic hydrological changes were caused mainly by earthquakeinduced surface phenomena and that there was little contribution from hydrothermal
fluid.

Keywords: Kumamoto earthquake, Hydrological changes, Streamflow, Spring,
Groundwater, Water-holding capacity

Graphical abstract*Corresponding author: Naoji Koizumi, koizumi.n@ses.usp.ac.jp

FULL PAPER

Open Access

Elastic wave velocity and electrical conductivity in abrine-saturated rock and microstructure of pores
Tohru Watanabe*, Miho Makimura, Yohei Kaiwa, Guillaume Desbois, Kenta Yoshida
and Katsuyoshi Michibayashi
Earth, Planets and Space 2019, 71:129 DOI: 10.1186/s40623-019-1112-9
Received: 29 July 2019, Accepted: 21 November 2019, Published: 29 November 2019

Abstract
Elastic wave velocity and electrical conductivity in a brine-saturated granitic rock were measured under confining pressures of up
to 150 MPa and microstructure of pores was examined with SEM on ion-milled surfaces to understand the pores that governelectrical conduction at high pressures. The closure of cracks under pressure causes the increase in velocity and decrease in
conductivity. Conductivity decreases steeply below 10 MPa and then gradually at higher pressures. Though cracks are mostly
closed at the confining pressure of 150 MPa, brine must be still interconnected to show observed conductivity. SEM observationshows that some cracks have remarkable variation in aperture. The aperture varies from ~ 100 nm to ~ 3 μm along a crack. FIB–
SEM observation suggests that wide aperture parts are interconnected in a crack. Both wide and narrow aperture parts work
parallel as conduction paths at low pressures. At high pressures, narrow aperture parts are closed but wide aperture parts are still
open to maintain conduction paths. The closure of narrow aperture parts leads to a steep decrease in conductivity, since narrow
aperture parts dominate cracks. There should be cracks in various sizes in the crust: from grain boundaries to large faults. A crack
must have a variation in aperture, and wide aperture parts mustgovern the conduction paths at depths. A simple tube model was
employed to estimate the fluid volume fraction. The fluid volume
fraction of 10−4–10−3 is estimated for the conductivity of 10−2 S/m.
Conduction paths composed of wide aperture parts are consistent
with observed moderate fluctuations (< 10%) in seismic velocity in thecrust.

Keywords: Electrical conductivity, Seismic velocity, Fluid, Crack,
Crust

Graphical abstract*Corresponding author: Tohru Watanabe, twatnabe@sus.u-toyama.ac.jp
7Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

FULL PAPER

Open Access

Fault weakening caused by smectite swelling
Jun Kameda*, Masaoki Uno, Marianne Conin, Kohtaro Ujiie, Yohei Hamada and Gaku Kimura
Earth, Planets and Space 2019, 71:131 DOI: 10.1186/s40623-019-1108-5
Received: 25 June 2019, Accepted: 12 November 2019, Published: 2 December 2019

Abstract
The large slip along the shallow subduction interface during the 2011 Tohoku-Oki earthquake (Mw9.0) caused a huge tsunamithat struck the northeast coast of Honshu, Japan. The Integrated Ocean Drilling Program Expedition 343 JFAST program
revealed that the fault zone is composed primarily of smectite. Our swelling experiments using the fault material
demonstrated that the swelling pressure systematically increases with a decrease in sample porosity. Based on in situ porosity
estimations in the IODP borehole, the swelling pressure of the fault is as high as 8 MPa, which is comparable to the effective
normal stress at the drill site (~ 7 MPa). This also suggests that the modified effective confining pressure of the fault is quite
low or potentially zero, meaning that fault strength is governed mainly by cohesion rather than frictional strength. The fault
may therefore be intrinsically weak, which could enhance the coseismic displacement toward the trench when earthquake
slip propagates from depth.
1000

Keywords: 2011 Tohoku-Oki earthquake, Fault strength, Smectite,

0.8

2σ

re

ng

elli

100

sw

10

su
res

p

0.6
rmal stresseffecve no

1
ty

0.1
swelling pressure > effecve normal stress

0

0.5
0.4
0.3

porosi0.01

0.7

Porosity

Pressure (MPa)

Swelling

1σ

0.2
0.1

0
200 400 600 800 1000 1200 1400 1600 1800 2000

Depth (mbsf)

Graphical abstract*Corresponding author: Jun Kameda, kameda@sci.hokudai.ac.jp

FULL PAPER

Open Access

Temporal changes in the distinct scattered wave packets
associated with earthquake swarm activity beneaththe Moriyoshi-zan volcano, northeastern Japan
Yuta Amezawa*, Masahiro Kosuga and Takuto Maeda
Earth, Planets and Space 2019, 71:132 DOI: 10.1186/s40623-019-1115-6
Received: 1 July 2019, Accepted: 23 November 2019, Published: 2 December 2019

Abstract
We investigated temporal changes in the waveforms of S-coda from triggered earthquakes around the Moriyoshi-zan volcano in northeasternJapan. Seismicity in the area has drastically increased after the 2011 off the Pacific coast of Tohoku earthquake, forming the largest cluster to thenorth of the volcano. We analyzed distinct scattered wave packets (DSW) that are S-to-S scattered waves from the mid-crust and appeared
predominantly at the high frequency range. We first investigated the variation of DSW for event groups with short inter-event distances and highcross-correlation coefficients (CC) in the time window of direct waves. Despite the above restriction, DSW showed temporal changes in theiramplitudes and shapes. The change occurred gradually in some cases, but temporal trends were much more complicated in many cases. We also
found that the shape of DSW changed in a very short period of time, for example, within ~ 12 h. Next, we estimated the location of the origin of
the DSW (DSW origin) by applying the semblance analysis to the data of the temporary small-aperture array deployed to the north of the largest
cluster of triggered events. The DSW origin is located between the largest
cluster within which hypocentral migration had occurred and the lowvelocity zone depicted by a tomographic study. This spatial distribution
implies that the DSW origin was composed of geofluid-accumulated midway
in the upward fluid movement from the low-velocity zone to the earthquake
cluster. Though we could not entirely exclude the possibility of the effect of
the event location and focal mechanisms, the temporal changes in DSW
waveforms possibly reflect the temporal changes in scattering properties in
and/or near the origin. The quick change in DSW waveforms implies that fast
movement of geofluid can occur at the depth of the mid-crust.

Keywords: Coda waves, Scattering, Triggered earthquakes, Seismic

swarm, Geofluid, The 2011 off the Pacific coast of Tohoku
earthquake*Corresponding author: Yuta Amezawa, h19ds201@hirosaki-u.ac.jp
8Graphical abstract

Earth, Planets and Space

FULL PAPER

Open Access

Water distribution in quartz schists of the Sanbagawa Metamorphic Belt,
Japan: infrared spectroscopic mapping and comparison of
the calibrations proposed for determining water contents
Jun-ichi Fukuda* and Ichiko Shimizu
Earth, Planets and Space 2019, 71:136 DOI: 10.1186/s40623-019-1117-4
Received: 16 August 2019, Accepted: 23 November 2019, Published: 5 December 2019

t

-B

-B

lg

O

Keywords: Quartz, Water content, Infrared spectroscopy, Calibration, Sanbagawa Metamorphic Belt

Abt

Water content (ppm H2O)

We evaluated water distributions in deformed quartz in schists along the Asemi River, Central Shikoku, in the Sanbagawa Metamorphic Belt,
Japan, using infrared spectroscopic (IR) mapping. The water trapped in quartz as molecular H2O showed a broad IR absorption band at 2800–
3750 cm−1. A necessary step before assessing the quartz water content was to evaluate and compare six previously proposed IR calibrations interms of the molar absorption coefficients of H2O (L/mol H2O cm2). The coefficients vary from 24,100
Water content
to 89,000 L/mol H2O cm2, and the values of the coefficients show a rough increase with increasing
Nakashima et al. 1995 Grain size
component of structural –OH in the IR spectra. We used Paterson’s calibration, which does not(point)Type I
require input regarding the mineral species, but which was modified in his paper for measurements
This Study(range)Type III
of molecular H2O in quartz. The absorption coefficient is 38,000 L/mol H2O cm2. IR mapping was
1200
180
performed on Sanbagawa metamorphic rocks with increasing grades of metamorphism, where the160mean grain size of quartz increases from ~ 40 to ~ 120 µm. The absorption bands that are only from1000
140
the quartz can be distinguished on the basis of microstructural observations and the corresponding
800
120
mapping results. The IR spectra of quartz commonly show dominant molecular H2O bands at
100
2800–3750 cm−1 with no additional bands associated with crystalline –OH when only quartz is
600
80
measured. The water contents of quartz in all our samples were 40–310 ppm, and these values are
400
60
about one-third of previously reported values measured using point analyses with the unified
40
Paterson’s calibration. This difference seems to reflect the incorporation of phyllosilicates in previous
200
20
measurements that showed a broad band around 3600 cm−1. The lowest and highest water00contents in our quartz samples are associated with intragranular water and grain boundary water,
024
6
8 10 12 14
Chl
Grt
Grt
respectively. We estimated the grain boundary widths to be at most ~ 10 nm on the basis of theBtwater contents at grain boundaries.
Ab-

Grain size (µm) from Y&T02

Abstract

Distance (km)Graphical abstract*Corresponding author: Jun-ichi Fukuda, jfukuda@crystal.kobe‑u.ac.jp

FULL PAPER

Open AccessFully dynamic earthquake sequence simulation of a fault in a viscoelastic
medium using a spectral boundary integral equation method:
does interseismic stress relaxation promote aseismic transients?
Yuki Miyake and Hiroyuki Noda*
Earth, Planets and Space 2019, 71:137 DOI: 10.1186/s40623-019-1113-8
Received: 12 July 2019, Accepted: 23 November 2019, Published: 5 December 2019

Abstract
Along subduction interfaces or some major faults, a seismogenic layer in the upper crust is underlain by a zone of slow-slip events (SSEs) andtremors, and seismicity disappears at greater depths. The transition between seismogenic and aseismic behavior may be caused by changes in
the frictional properties of the fault or changes in viscoelastic properties of the surrounding medium. Although aseismic transients have been
numerically generated in previous studies by changing the frictional properties and compared to SSEs, the effect of viscoelasticity on thetransition remains to be studied. In this study, we implemented interseismic viscoelastic stress relaxation in a simulation code for twodimensional antiplane fully dynamic earthquake sequences in a uniform elastic material based on a spectral boundary integral equation method.
In the implementation, we developed a suitable algorithm in which the viscoelastic relaxation is calculated by evolution of an “effective slip,”which gives viscoelastic stress change on the fault if convolved with a static Green’s function. We conducted parametric studies for a fault with arate-weakening patch on two parameters: viscoelastic relaxation time tc and characteristic length of the state evolution in the rate- and statedependent friction law L. The behavior of the simulated fault can be classified into fourclasses, earthquakes (EQ), aseismic transients (AT), stable sliding (SS), and stuck (ST), in which
the central part of the rate-weakening patch has a diminishingly small slip rate and ispermanently locked. A phase diagram of the fault behavior shows that there are two different
types of seismogenic–aseismic transition. As L increases, an EQ patch changes to an AT patch
before becoming an SS patch, as has been reported in previous studies in an elastic limit. The
boundary between AT and SS can be explained via linear stability analysis of a systemcomposed of a spring, a dashpot, and sliders. As tc decreases, the recurrence interval of theearthquakes diverges, and an EQ patch changes to an ST patch unless L is within a narrow
range. Therefore, the transition associated with SSEs and tremors is dominated by changes infrictional properties rather than changes in viscoelastic properties.

Keywords: Earthquake sequence simulation, Viscoelasticity, Spectral boundary integral
equation method, Memory variable method*Corresponding author: Hiroyuki Noda, noda.hiroyuki.7z@kyoto-u.ac.jp
9Graphical abstract

Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

FULL PAPER

Open AccessContribution to crustal strain accumulation of minor faults:
a case study across the Niigata–Kobe Tectonic Zone, Japan
Tomonori Tamura*, Kiyokazu Oohashi, Makoto Otsubo, Ayumu Miyakawa and Masakazu Niwa
Earth, Planets and Space 2020, 72:7 DOI: 10.1186/s40623-020-1132-5
Received: 3 October 2019, Accepted: 8 January 2020, Published: 20 January 2020

Abstract
Recent global navigation satellite system (GNSS) data for the Japanese Islands have revealed a high-strain-rate region suggesting
the existence of a region of broad-scale crustal deformation. The Niigata–Kobe Tectonic Zone (NKTZ), which is the high-strainrate zone in central Japan, shows a short-term dextral strain rate of ~ 12 mm/year. The total slip rate of the Quaternary fault zones
in the NKTZ has been estimated as ~ 6.7 mm/year, accounting for just over half the short-term strain rate of the zone. However,this slip rate underestimates the total slip rate on faults within the NKTZ owing to possible distributed deformation on minor
faults. This study quantifies the slip rate attributable to these other faults in the southeastern-central NKTZ and reveals theunique deformation structure across the high-strain-rate zone, which comprisesa Quaternary fault core, a Quaternary fault damage zone, an incipient brittle
shear zone (active background), and an inactive background. The spatial
characteristics of the incipient brittle shear zone can be explained in terms offault density, which increases toward the central NKTZ. Minor faults located> 500 m from major Quaternary faults but within the NKTZ have sense of shear
consistent with that of the major faults. In contrast, minor faults outside of the
NKTZ show sense of shear that differ from the dextral displacement of thehigh-strain-rate zone and do not contribute to the slip rate of the zone. The
total slip rate of minor faults in the southeastern-central NKTZ is estimated tobe 0.46–2.88 mm/year (roughly equal to a major Quaternary fault in the zone),which implies 4–24% of crustal strain is stored in the active background.

Keywords: Niigata–Kobe Tectonic Zone, Strain-rate paradox, Minor faults,
Brittle shear zone, Slip rate

Graphical abstract*Corresponding author: Tomonori Tamura, i004wb@yamaguchi-u.ac.jp

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Open Access

Simple kinetic model for replacement reactions involving
solid solutions: the significant role of geofluids
Tatsu Kuwatani* and Mitsuhiro Toriumi
Earth, Planets and Space 2020, 72:8 DOI: 10.1186/s40623-020-1135-2
Received: 29 August 2019, Accepted: 15 January 2020, Published: 3 February 2020

Abstract
We develop a kinetic model that illustrates the relative role of intergranular diffusion and surface reactions in overall metamorphic
net-transfer reactions involving solid solutions. The behavior and state of the system were observed to depend on the non-dimensionalparameter(s), defined herein by γ≡ (k · L · Kn−1)/Deff, where k denotes the dissolution rate constant of the mineral, L denotes the distance
between the two minerals participating in the reaction, K denotes the coefficient of the equilibrium abundance ratio of the precipitatedcomponent in the intergranular fluid to those in the mineral, n denotes the order of the dissolution reaction, and Deff denotes theeffective diffusion coefficient of the precipitated component in the intergranular fluid. Whenγ is small, the system becomeshomogeneous, which is controlled by the surface
Xreaction. In contrast, a largeγ implies a
Heterogeneous
heterogeneous system controlled by both surface
Surface+Diffusion
reaction and diffusion. Geofluid is among the most-controlled
Inactive
important influencers of kinetic regimes andmaterial exchange
reaction textures in metamorphic and metasomatic
rocks because the intergranular fluid distribution
X
C
(X )
γ
γ
considerably affects the intergranular diffusivity(X )eff(D ). Our conceptual model can potentially reveal
the nature of the reaction kinetics and the fluid–rock interactions and capture the complex
Homogeneous(close to composition A)
behaviors of natural systems in future extensions.
Surface B-controlled
A

δ=

1

δ=

0.9

2

10

A

6

-2

10

-4

10

10

δ=

δ=
10

6

om
og
en
ac
eo
e
A
,B us
m
-c
at
on
er
t
ia
l e roll
e
xc
ha d
ng
e

0
B

e

H

B

α

0

rf

Su

10 -4

α

α

1

iv

ct

Active material exchange

A
10 -4

δ=

δ=

A

B

4

α

10

0.7

K (1)

K (1)

B

δ=

10
4

0.5

0.9

Keywords: Kinetics, Fluid–rock interaction, Solidsolution, Geofluids, Metasomatism

10

-4

10

δ=

δ=

2

δ=
10

10 2

0.1

0.3

1

10-2

-2

-6

10

-2

10 4

δ=
10

-4

10

δ=

=1
0 -6

Active material exchange

0.7

δ=

B

Surface A-controlled

Homogeneous(close to composition B)δ

10 2

0.3 0.5

1

α

0.1

10 4

10 -2

1

γA

10 2

10 4

10-4

Graphical abstract*Corresponding author: Tatsu Kuwatani, kuwatani@jamstec.go.jp
10

10-2

1

γA

102

104

Earth, Planets and Space

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Open Access

Identification of a nascent tectonic boundary in the San-in area,
southwest Japan, using a 3D S-wave velocity structure
obtained by ambient noise surface wave tomography
Yudai Suemoto, Tatsunori Ikeda, Takeshi Tsuji* and Yoshihisa Iio
Earth, Planets and Space 2020, 72:15 DOI: 10.1186/s40623-020-1139-y
Received: 27 September 2019, Accepted: 18 January 2020, Published: 5 February 2020

Abstract
We derived a three-dimensional S-wave velocity model for the San-in area ofsouthwest Japan to examine heterogeneous structures such as tectonic faults.
Many earthquakes occur in this area, but much of the activity has been relatively
recent, so the fault distribution has yet to be fully clarified. Here, we used
continuous ambient noise data from a dense seismic network, deployed from
November 2009 to extract Rayleigh and Love wave dispersion data between
station pairs, and then applied a direct surface wave inversion to the phasevelocities of each station pair to determine a three-dimensional S-wave velocity
model. In the resulting model, faults and a previously unrecognized tectonic
boundary appeared as low-velocity anomalies or velocity boundaries, and thevelocity anomalies were also associated with many past earthquake
hypocenters. These results contribute to our understanding of heterogeneous
structures caused by recent tectonic motion and of possible future tectonic
activity, such as intraplate earthquakes. Surface wave tomography using
ambient noise recorded in dense seismic networks could also be applied inother parts of the world to reveal new heterogeneous geological structures (i.e.,
unrevealed tectonic faults) and could contribute to disaster mitigation.
Keywords: Ambient noise, Surface wave tomography, Nascent tectonic
boundary, 2000 western Tottori earthquake, San-in shear zone
Graphical abstract*Corresponding author: Takeshi Tsuji, tsuji@mine.kyushu-u.ac.jp

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Open Access

Detection of plastic strain using GNSS data of pre- andpost-seismic deformation of the 2011 Tohoku-oki earthquake
Yukitoshi Fukahata*, Angela Meneses-Gutierrez and Takeshi Sagiya
Earth, Planets and Space 2020, 72:18 DOI: 10.1186/s40623-020-1144-1
Received: 30 August 2019, Accepted: 24 January 2020, Published: 13 February 2020

Abstract
In general, there are three mechanisms causing crustal deformation: elastic, viscous, andplastic deformation. The separation of observed crustal deformation to each component
has been a challenging problem. In this study, we succeed in separating plastic deformation
as well as viscous deformation in the northern Niigata–Kobe Tectonic Zone (NKTZ), central
Japan, using GNSS data before and after the 2011 Tohoku-oki earthquake, under theassumptions that elastic deformation is principally caused by the plate coupling along theJapan trench and that plastic deformation ceased after the Tohoku-oki earthquake due tothe stress drop caused by the earthquake. The cessation of plastic deformation can beunderstood with the concept of stress shadow used in the field of seismic activity. The
separated strain rates are about 30 nanostrain/year both for the plastic deformation in thepreseismic period and for the viscous deformation in both the pre- and post-seismic
periods, which means that the inelastic strain rate in the northern NKTZ is about 60 and 30
nanostrain/year in the pre- and post-seismic periods, respectively. This result requires therevision of the strain-rate paradox in Japan. The strain rate was exceptionally faster before
the Tohoku-oki earthquake due to the effect of plastic strain, and the discrepancy between
the geodetic and geologic strain rates is much smaller in usual time, when the plastic strain
is off. In order to estimate the onset timing of plastic deformation, the information on stresshistory is essentially important.

Keywords: Plastic strain, Inelastic strain, NKTZ, Tohoku-oki earthquake, ∆CFF, Strain-rate
paradoxGraphical abstract*Corresponding author: Yukitoshi Fukahata, fukahata@rcep.dpri.kyoto‑u.ac.jp
11Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

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Open Access(U–Th)/He thermochronometric mapping across the northeast
Japan Arc: towards understanding mountain building in an
island-arc setting
Shoma Fukuda*, Shigeru Sueoka, Barry P. Kohn and Takahiro Tagami
Earth, Planets and Space 2020, 72: 24 DOI: 10.1186/s40623-020-01151-z
Received: 29 September 2019, Accepted: 11 February 2020, Published: 27 February 2020

Abstract
Topographic relief in arc-trench systems is thought to be formed by plate subduction;
however, few quantitative investigations have so far been reported, with respect to therelated mountain building process. This study applies apatite and zircon (U–Th)/He
thermochronometry (AHe, ZHe, respectively) to Cretaceous granite rocks in the northpart of the northeast (NE) Japan Arc to reveal its cooling/denudation history. Weighted
mean AHe ages ranging from 88.6 to 0.9 Ma and ZHe ages from 83.9 to 7.4 Ma were
determined for 10 rock samples. Using the AHe data, denudation rates were obtained foreach sample. On the fore-arc side, denudation rates of < 0.05 mm/year were calculated,indicating a slow denudation process since the Paleogene. However, in the Ou Backbone
Range and on the back-arc side, denudation rates at > 0.1–1.0 mm/year were computed,
probably reflecting a recent uplift event since ~ 3–2 Ma. These data indicate a clear
contrast in thermal and denudation histories between the tectonic units in this study
area, similar to that previously reported from the southern part of NE Japan Arc. A
comparison of the thermal/denudation histories between the N- and S- traverses,
revealed the arc-parallel trend, the uplift model of the volcanic arc, and some minor
variations of thermal/denudation histories in each tectonic unit. This study offers somefurther insights into the understanding of tectonic processes in an island-arc setting.

Keywords: Low-temperature thermochronology, Arc-trench system, Northeast Japan
Arc, (U–Th)/He thermochronometry, Thermal/denudation histories

Graphical abstract*Corresponding author: Shoma Fukuda, s-fukuda@kueps.kyoto-u.ac.jp

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Open Access

Structural control and system-level behavior of the seismic
cycle at the Nankai TroughQibin Shi, Sylvain Barbot*, Shengji Wei, Paul Tapponnier, Takanori Matsuzawa and Bunichiro Shibazaki
Earth, Planets and Space 2020: 72:27 DOI: 10.1186/s40623-020-1145-0
Received: 3 October 2019, Accepted: 24 January 2020, Published: 2 March 2020

Abstract
The Nankai Trough in Southwest Japan exhibits a wide spectrum of fault slip, with long-term and short-term slow-slip events,
slow and fast earthquakes, all associated with different segments down the plate interface. Frictional and viscous properties varydepending on rock type, temperature, and pressure. However, what controls the down-dip segmentation of the Nankai
subduction zone megathrust and how the different domains of the subduction zone interact during the seismic cycle remains
unclear. Here, we model a representative cross-section of the Nankai subduction zone offshore Shikoku Island where thefrictional behavior is dictated by the structure and composition of the overriding plate. The intersections of the megathrust withthe accretionary prism, arc crust, metamorphic belt, and upper mantle down to the asthenosphere constitute important domainboundaries that shape the characteristics of the seismic cycle. The
mechanical interactions between neighboring fault segments and theimpact from the long-term viscoelastic flow strongly modulate therecurrence pattern of earthquakes and slow-slip events. Afterslip
penetrates down-dip and up-dip into slow-slip regions, leading toaccelerated slow-slip cycles at depth and long-lasting creep waves in
the accretionary prism. The trench-ward migrating locking boundary
near the bottom of the seismogenic zone progressively increases thesize of long-term slow-slip events during the interseismic period. Fault
dynamics is complex and potentially tsunami-genic in the accretionary
region due to low friction, off-fault deformation, and coupling with theseismogenic zone.
p

ne

zo

sli

lip

w-

ws

rm

p

ee

th
e

rm

t-t
e

or

Iso

Depth (km)

C

slo

slo

rm

-te

ng

ab

Lo

duct B
ing
sl

Cr

en

ism
og

Se

sub

100

D

Mantleasthenowedge
sphereE

Ocea
astheno nic
spheC(n oupocr led
ee
p)

200

re

0

1019 1021 1023
Effective viscosity(Pa s)

Keywords: Friction, Rheology, Subduction zones

200
e fromtrenchDistanc(km)

400Graphical abstract*Corresponding author: Sylvain Barbot, sbarbot@usc.edu
12back-arc basin(Sea of Japan)

p

ic

ow

all

Sh

A

Sh

ch

rr
ise

sli

w-

slo

te

Ab

0

Seto
Media
Tectonic nInlandLine
Sea
Shikokust (cons
Honshuolibaseme dated
nt)
Mantle wedge corner(metamorphic belt)
Arc low
Mantle er crustlid (perid
otite)

Arc cru

Tr
en

lh

sa

ys

Ou

ills

Accretionary prism (clays)

Earth, Planets and Space

FULL PAPER

Open AccessPreparation zones for large crustal earthquakes
consequent on fault-valve action
Richard H. Sibson
Earth, Planets and Space 2020, 72:31 DOI: 10.1186/s40623-020-01153-x
Received: 25 August 2019, Accepted: 18 February 2020, Published: 4 March 2020

Abstract
A combination of geological evidence (in the form of hydrothermal vein systems in exhumed fault systems) and geophysical information around
active faults supports the localized invasion of near-lithostatically overpressured aqueous fluids into lower portions of the crustal seismogenic
zone which commonly extends to depths between 10 and 20 km. This is especially the case for compressional–transpressional tectonic regimes
which, beside leading to crustal thickening and dewatering through prograde metamorphism, are also better at containing overpressure and are‘load-strengthening’ (mean stress rising with increasing shear stress), the most extreme examples being associated with areas undergoing active
compressional inversion where existing faults are poorly oriented for reactivation. In these circumstances, ‘fault-valve’ action from ascending
overpressured fluids is likely to be widespread with fault failure dual-driven by a combination of rising fluid pressure in the lower seismogenic
zone lowering fault frictional strength, as well as rising shear stress. Localized fluid overpressuring nucleates ruptures at particular sites, but
ruptures on large existing faults may extend well beyond the regions of intense overpressure. Postfailure, enhanced fracture along fault rupturezones promotes fluid discharge through the aftershock period, increasing fault frictional strength
before hydrothermal sealing occurs and overpressures begin to reaccumulate. The association of
rupture nucleation sites with local concentrations of fluid overpressure is consistent with selective
invasion of overpressured fluid into the roots of major fault zones and with observed non-uniform
spacing of major hydrothermal vein systems along exhumed brittle–ductile shear zones. A rangeof seismological observations in compressional–transpressional settings are compatible with thishypothesis. There is a tendency for large crustal earthquakes to be associated with extensive(L ~ 100–200 km) low-velocity zones in the lower seismogenic crust, with more local Vp/Vs
anomalies (L ~ 10–30 km) associated with rupture nucleation sites. In some instances, theselow-velocity zones also exhibit high electrical conductivity. Systematic, rigorous evaluation isneeded to test how widespread these associations are in different tectonic settings, and to see
whether they exhibit time-dependent behaviour before and after major earthquake ruptures.

Keywords: Compressional–transpressional faulting, Fluid overpressures, Gold transport,

Lower seismogenic zone, Dual-driven failure, Fault-valve action, Low-velocity zones,
Vp/Vs anomalies

Graphical abstract

Corresponding author: Richard H. Sibson, rick.sibson@otago.ac.nz

FULL PAPER

Open Access

Dynamic rupture simulation of 2018, Hokkaido Eastern
Iburi earthquake: role of non-planar geometry
Tatsuya Hisakawa*, Ryosuke Ando, Tomoko Elizabeth Yano and Makoto Matsubara
Earth, Planets and Space 2020, 72:36 DOI: 10.1186/s40623-020-01160-y
Received: 3 September 2019, Accepted: 3 March 2020, Published: 16 March 2020

Abstract
The 2018, Hokkaido Eastern Iburi, Japan, earthquake is an event characterized by complexity of the rupture process and slip
pattern, which may involve both reverse and strike-slip motion depending on the locations on the fault surface. We perform
dynamic rupture simulations based on simple physical laws, conditions for stressing and fault friction, and the non-planar
fault geometry constrained by the aftershock observation. The complex fault geometry is numerically treated by theboundary integral equation method accelerated by the fast domain portioning method. The fault geometry is characterizedprimarily by the combination of six fault planes. As a result, we are able to explain several observed features of the event,including the spatial variation of the final fault slip and rupture
velocity, which are inferred from the kinematic slip inversion. We
also succeed in refining the constraint of the regional stress fieldin the focal area based on the simulation. Our results show thatthe overall patterns of the complex rupture event can be
reproduced by a relatively simple model of the regional stressand the fault friction, if the geometrical complexity of the fault isproperly taken into account.

Keywords: 2018 Hokkaido Iburi Eastern earthquake, Fault
geometry, Dynamic rupture, Fast domain
partitioning BIEM

Graphical abstract*Corresponding author: Tatsuya Hisakawa, hisakawa@eps.s.u-tokyo.ac.jp
13Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

FULL PAPER

Open Access

Deep groundwater discharge after the 2011 Mw 6.6 Iwaki
earthquake, Japan
Tsutomu Sato*, Kohei Kazahaya, Norio Matsumoto and Masaaki Takahashi
Earth, Planets and Space 2020, 72:54 DOI: 10.1186/s40623-020-01181-7
Received: 12 September 2019, Accepted: 23 April 2020, Published: 1 May 2020

Abstract
Hot spring discharge was linked to the 2011 Mw 6.6 Iwaki earthquake. Periodic surveys revealed that the discharge continued
for more than 7 years, which is a rare and valuable long-term record of hot spring discharge triggered by an earthquake in a
non-volcanic area. In terms of coseismic changes, based on a comparison of the spatial distribution of changes in thecoseismic water head and calculated crustal volumetric strain using
a fault model, hot spring water discharge was found to be caused
by a change in the coseismic crustal volumetric strain. As for thepostseismic changes, observations over 7 years revealed a gradualrise in the temperature and chloride ion concentration of the hotspring water. Such long-term hot spring discharge may be
explained by the following two causes: the rise of thermal waterfrom the deep part and the permeability changes along the hotspring channels.

Keywords: Coseismic hydrological change, Hot spring water

discharge, 2011 Iwaki earthquake, 2011 Tohoku-oki
earthquake, Crustal strain, Water temperature, Chloride
ion, Sulfate ion, Oxygen stable isotopic ratio
Graphical abstract*Corresponding author: Tsutomu Sato, sugar@ni.aist.go.jp

FULL PAPER

Open Access

Inelastic strain in the hypocentral region of the 2000 Western
Tottori earthquake (M 7.3) inferred from aftershock seismic
moment tensors
Satoshi Matsumoto*, Yoshihisa Iio, Shinichi Sakai and Aitaro Kato
Earth, Planets and Space 2020, 72:62 DOI: 10.1186/s40623-020-01186-2
Received: 26 August 2019, Accepted: 24 April 2020, Published: 11 May 2020

Abstract

Depth (km)Inelastic deformation due to seismic activity is an important signal that reflects fault evolution. In particular, aftershock sequencesindicate the evolution of damage in a medium that has experienced a large earthquake. Herein, we discuss the inelastic strain rate
surrounding the fault that produced the M 7.3 Western Tottori earthquake in 2000 using long-term aftershock analysis. To obtain highresolution focal mechanisms 18 years after the earthquake occurrence, we conducted dense seismic observations in the focal area. The
inelastic strain rate estimated from the aftershock seismic moment tensor data showed spatial variations within a range of 10−7–10−11 per
year, 18 years after the main shock. By comparing the inelastic strain rates from immediately after the earthquake and 18 years later, wedetected the increase in the spatial variations in the inelastic strain rate; the variations are as small as 102 (= 10−5/10−7) for the early stage
but as large as 104 (= 10−7/10−11) for the later period. In addition, the decay
N30 W
S30 E
of the rate during these two periods varied spatially from spatial bin to bin.
0
Certain bins in the northern segment of the earthquake fault, southernedge of the fault, and surrounding the location of the preceding swarm
activity to the M 7.3 event showed slower decay rates than the inverse of
the lapse time since the occurrence of the M 7.3 earthquake. We modeled
this decay rate change as the relaxation response of a power-law fluid to
Magnitude
an elastic strain input from the large earthquake. Most parts of the fault
10
5.0
can be explained by this model. However, the areas with low decay rates
3.0
suggest the presence of a dragging mechanism, such as aseismic slip, at or
1.0
around these locations.
P -value

Keywords: Inelastic strain, Fault development, Seismic observation,2000 Western Tottori earthquake

b

S30 E
0

15 Oct. 2000 – 30 Nov. 2000

Depth (km)*Corresponding author: Satoshi Matsumoto, matumoto@sevo.kyushu-u.ac.jp
1410Graphical
abstract
N30 W S30 E

Mar. 2017 – Apr. 2018

N30 W

Earth, Planets and Space

FULL PAPER

Open AccessGeological structures controlled the rupture process of the2011 M9.0 Tohoku-Oki earthquake in the Northeast Japan Arc
Kei Baba and Takeyoshi Yoshida*
Earth, Planets and Space 2020, 72:94 DOI: 10.1186/s40623-020-01212-3
Received: 26 August 2019, Accepted: 1 June 2020, Published: 8 July 2020

Abstract
By interpreting the 2D/3D seismic survey data acquired in the surrounding ocean areas of the Northeast (NE) Japan Arc, we clarified thedetailed geological structure and demonstrated that the basic structure in the hanging-wall plate of the subduction system consists ofmany structural blocks (segments) separated by NW–SE trending large transcurrent faults (strike-slip faults). This structural configuration
showed a close relationship with the distribution of foreshocks, mainshock, and aftershocks, coseismic slip models of the 2011 M9.0
Tohoku-Oki megathrust earthquake, coseismic slip area of M-7 class earthquakes, quasi-static slip rates, back slip rate, and seismic
tomography images. In addition, the coseismic slip models
revealed that the trenchward forearc of the structural blocks
between the Offshore Hidaka tectonic line and the HonjoSendai tectonic line fitted well with the coseismic slip areaof the 2011 Tohoku-Oki earthquake. These findings suggestthat the structural blocks bounded by these two tectonic
lines slipped rapidly trenchward when the mainshock
occurred. The M7 earthquakes were also concentrated
along these two tectonic lines, thereby suggesting a close
relationship between seismic activity and the inherited
geological structure of the overriding plate in the NE Japan
forearc.

Keywords: 2011 Tohoku-Oki earthquake, Transcurrent

fault, Rupture process, Northeast Japan Arc,
Hypocenter distribution, Segment boundary,
Plate boundary megathrustFig. 1

Fig.3b*Corresponding author: Takeyoshi Yoshida, takeyoshi.yoshida.a1@tohoku.ac.jp

FULL PAPER

Open AccessOriginal composition and formation process of slab-derived
deep brine from Kashio mineral spring in central Japan
Fumitake Kusuhara*, Kohei Kazahaya, Noritoshi Morikawa, Masaya Yasuhara, Hidemi Tanaka,
Masaaki Takahashi and Yuki Tosaki
Earth, Planets and Space 2020, 72:107 DOI: 10.1186/s40623-020-01225-y
Received: 9 September 2019, Accepted. 30 June 2020, Published: 23 July 2020

Abstract
Brine samples from the wells in the Kashio mineral spring (an “Arima-type” hot spring at Ooshika-Mura, central Japan) were analyzed todetermine the original chemical and isotopic compositions of the deep brine end-member before its dilution by meteoric water and to
elucidate the origin of the end-member. The trends of variation between Cl, δD, and δ18O indicated the existence of a two-component
mixing system and a systematic variation in the mixing ratio, which were mentioned in previous studies. By carefully tracking thevariation in tritium (3H) and atmospheric noble gas in the brine, the Cl concentration in the end-member was determined to be 24,000
mg/L. This value is consistent with the result of previous studies. Based on the estimated composition and other related data, we inferredthat the end-member originated from slab-derived fluid, which in turn may haveundergone oxygen isotope exchange reactions with minerals. Although both
the Arima and Kashio brines are considered to be derived from fluid dehydrated
from the Philippine Sea slab, the chemical and isotopic compositions of theKashio end-member are different from those of the Arima end-member. In
particular, the Kashio end-member is characterized by low Cl concentration(~ 40% lower than that in the Arima end-member), low hydrogen isotope ratio,
and low 3He/4He ratio (1.4 Ra). These results indicate that the chemical andisotopic compositions of the slab-derived fluid are different for each location.
The significant difference in δD could reflect the difference in the dehydration
depth. Finally, the low temperature and relatively low 3He/4He ratio of the brine
end-member could be explained by its long residence time within the crust.

Keywords: Kashio mineral spring, Arima-type hot spring, Slab-derived fluid,
Chemical and isotopic composition, Tritium, Noble gas*Corresponding author: Fumitake Kusuhara, kusuhara@criepi.denken.or.jp
15Graphical abstract

Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

EXPRESS LETTER

Open AccessOrigin of the seismic belt in the San-in district, southwest
Japan, inferred from the seismic velocity structure of
the lower crust
Hiroo Tsuda*, Yoshihisa Iio and Takuo Shibutani
Earth, Planets and Space 2019, 71:109 DOI: 10.1186/s40623-019-1091-x
Received: 3 July 2019, Accepted: 16 October 2019, Published: 25 October 2019

Abstract
A long linear distribution of epicenters is seen along the Japan Sea coast in the San-in district located in southwestern Japan. This linear
distribution of epicenters is called the seismic belt in the San-in district. The localization of intraplate earthquakes in the San-in district,
far from plate boundaries, is not well understood. To answer this question, we look at the seismic velocity structure of the lower crust
beneath the San-in district using seismic travel-time tomography. Our results show the existence of a low-velocity anomaly in the lower
crust beneath the seismic belt. We infer that the deformation was concentrated in the low-velocity zone due to compressive stresscaused by the subduction of oceanic plates, that stress concentration occurred just above the low-velocity zone, and that the seismic
belt was therefore formed there. We also calculated the cutoff depths of shallow
intraplate earthquakes in the San-in district. Based on the results, we consider the
possible causes of the low-velocity anomaly in the lower crust beneath the seismic belt.
We found that the cutoff depths of the intraplate earthquakes were shallower in theeastern part of the low-velocity zone in the lower crust beneath the seismic belt anddeeper in the western part. Thus, the eastern part is likely to be hotter than the western
part. We inferred that the eastern part was hot because a hot mantle upwelling
approaches the Moho discontinuity below it and the resulting high temperature may be
the main cause of the low-velocity anomaly. On the other hand, in the western part, weinferred that the temperature is not high because the mantle upwelling may not exist atshallow depth, and water dehydrated from the Philippine Sea plate reaches the lower
crust, and the existence of this water may be the main cause of the low-velocity anomaly.

Keywords: Seismic belt, San-in district, Travel-time tomography, Lower crust,Low-velocity zone, Cutoff depth of earthquakes

Graphical abstract*Corresponding author: Hiroo Tsuda, tsuda.hiroo.87x@st.kyoto-u.ac.jp

EXPRESS LETTER

Open Access

Inelastic deformation zone in the lower crust for the San-in
Shear Zone, Southwest Japan, as observed by a dense
GNSS network
Angela Meneses-Gutierrez* and Takuya Nishimura
Earth, Planets and Space 2020, 72:10 DOI: 10.1186/s40623-020-1138-z
Received: 31 August 2019, Accepted: 18 January 2020, Published: 3 February 2020

Abstract
We analyze Global Navigation Satellite System (GNSS) within the San-in Shear
Zone (SSZ), Southwest Japan, in order to clarify the width of the inelastic
deformation zone in the lower crust beneath it. We assumed multiple discrete
fault dislocations distributed below the seismogenic depth in a channel torepresent inelastic deformation in the lower crust. Kinematic models at 68%
confidence level at three profiles indicate that deformation can be explained bya deep inelastic deformation zone with a width of 0.5–56.0 km, 0.5–79.5 km, and
0.0–58.5 km, for the Eastern, Central and Western profiles, respectively; and a
relative moving rate between the two blocks sandwiching the zone of 6.2–8.0
mm/year. In Eastern Tottori, the center of the estimated inelastic zone coincides
with the source region of the 1943 Tottori earthquake. In Central and Western
Tottori, the channel is in agreement with the source regions of the 2000 Westernand the 2016 Central Tottori earthquake. Current GNSS network provides alimited contribution to constraining the width of the deformation in the lower
crust.

Keywords: San-in Shear Zone, GNSS, Intraplate deformation, Strain
concentration

Graphical abstract*Corresponding author: Angela Meneses-Gutierrez, angela@seis.nagoya‑u.ac.jp
16Earth, Planets and Space

EXPRESS LETTER

Open Access

Inelastic behavior and mechanical strength of the shallow
upper crust controlled by layer-parallel slip in thehigh-strain zone of the Niigata region, Japan
Makoto Otsubo*, Ikuo Katayama, Ayumu Miyakawa and Takeshi Sagiya
Earth, Planets and Space 2020, 72:30 DOI: 10.1186/s40623-020-01154-w
Received: 27 September 2019, Accepted: 22 February 2020, Published: 4 March 2020

Abstract
We investigated the relationship between contractional deformation of sedimentary mass in the upper crust and the geodetic
strain rate in a high-strain zone (part of the Niigata–Kobe Tectonic Zone) of the mid-Niigata region, Central Japan. We
observed numerous examples of layer-parallel slip (bedding-plane slip) generated by folding. The slip layers have an average
spacing of ~ 3.6 m (measured normal to bedding planes) at the
Katagai site. In shallow sedimentary units of the upper crust,
numerous bedding-plane slip events act to reduce themechanical strength (effective elastic thickness) of sedimentaryrocks under contractional deformation. The results support anexisting model, based on geodetic observations, of mechanicaldecoupling between the weak sedimentary layers andbasement. On the layer-parallel slip, we measured the frictiondcoefficient of gouge generated by bedding-plane slip and of
mud around non-slip surfaces using double-direct shear tests,
and found no difference in frictional coefficient between slip andnon-slip surfaces.

Keywords: Active fold, Bedding-plane slip, Buckling,

Deformation, Niigata–Kobe Tectonic Zone
Graphical abstract*Corresponding author: Makoto Otsubo, otsubo-m@aist.go.jp

EXPRESS LETTER

Open Access

Stationarity of aftershock activities of the 2016 Central Tottori
Prefecture earthquake revealed by dense seismic observation
Yoshihisa Iio*, Satoshi Matsumoto, Yusuke Yamashita, Shin’ichi Sakai, Kazuhide Tomisaka,
Masayo Sawada, Takashi Iidaka, Takaya Iwasaki, Megumi Kamizono, Hiroshi Katao, Aitaro Kato,
Eiji Kurashimo, Yoshiko Teguri, Hiroo Tsuda and Takashi Ueno
Earth, Planets and Space 2020, 72:42 DOI: 10.1186/s40623-020-01161-x
Received: 8 October 2019, Accepted: 8 March 2020, Published: 31 March 2020

Abstract
To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation networkwith 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock areaof the 2016 Central Tottori Prefecture earthquake. We determined the hypocenters and focal mechanisms of the aftershocks very
precisely in the period from October 22 to December 15. We then investigated the temporal changes in the spatial distributions of
hypocenters and T-axis azimuths of focal mechanisms. The distributions of aftershock hypocenters and T-axis azimuths are basicallytemporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid
decrease of aftershocks with T-axis azimuths of WSW to west was observed. If fluid rises from the lower crust due to fault rupture, thelocations of aftershocks and focal mechanisms
may change over time, especially in the deepest
part of the aftershock region. However, thetemporal change in these parameters was notapparent at depth. These observations suggestthat the aftershock activity of the Central Tottori
Prefecture earthquake was controlled mainly by
stress concentration rather than strength
reduction due to high fluid pressure.

Keywords: Intraplate earthquake, Aftershock,Stress, Strength, Fluid

Graphical abstract*Corresponding author: Yoshihisa Iio, iio@rcep.dpri.kyoto-u.ac.jp
17Crustal Dynamics: Toward Integrated View of Island Arc Seismogenesis

EXPRESS LETTER

Open AccessCoulomb stress change on inland faults during megathrustearthquake cycle in southwest Japan
Tsukasa Mitogawa* and Takuya Nishimura
Earth, Planets and Space 2020, 72:66 DOI: 10.1186/s40623-020-01174-6
Received: 30 September 2019, Accepted: 29 March 2020, Published: 13 May 2020

Abstract
In the subduction zone, megathrust earthquakes may modulate the shallow crustal seismicity in the overriding plate. Historical documents
indicate the frequent occurrence of large shallow crustal earthquakes in the overriding continental plate 50 years before and 10 years after themegathrust earthquakes along the Nankai trough in southwest Japan. In this study, we model megathrust earthquake cycles in a simple obliquesubduction zone considering the viscoelasticity, and calculate the temporal evolution of the Coulomb failure stress changes (CFS) on the crustal
faults in the overriding plate. Further, we examine the variation of CFS depending on the location and fault type, and the active period of crustal
earthquakes in which CFS exceeds the previous maximum. Our viscoelastic model suggests that the dependency of the active period on the
distance from the megathrust fault is less when the intrinsic loading rate of the inland fault is low. Moreover, it suggests that the viscoelastic
stress evolution on faults with negative coseismic CFS renders the active period longer or
shorter than those in a pure elastic medium. The temporal evolution of CFS on most major
active faults in southwest Japan can be categorized into two groups with the following different
characteristics: one is that CFS is positive coseismically and peaks 10 years after a megathrustearthquake. The other is that CFS is negative coseismically, and does not recover to thepreseismic one for more than 50 years after a megathrust earthquake. This can explain thetemporal sequence of the historical earthquakes in southwest Japan. Our model which includes
viscoelastic relaxation successfully expresses the activation of shallow crustal earthquakes in theoverriding continental plate not only before the megathrust earthquake, but also after. If theapparent frictional coefficient is less than ~ 0.1, the coseismic CFS on the source faults of the1943 Mj7.3 Tottori earthquake, 1596 M7.0 Keicho Iyo earthquake, and 1596 M7.5 Keicho Fushimi
earthquake that occurred within 10 years before the megathrust earthquake along the Nankai
trough is negative. Therefore, to explain the occurrence of these historical earthquakes, ourmodel suggests that the apparent frictional coefficient must be less than ~ 0.1.

Keywords: Megathrust earthquake cycle, Coulomb stress change, Viscoelasticity, Inland
fault, Southwest Japan

Graphical abstract*Corresponding author: Tsukasa Mitogawa, mitogawa.tsukasa.53w@st.kyoto‑u.ac.jp

TECHNICAL REPORT

Open Access

Real-time crustal monitoring system of Japanese Islands
based on spatio-temporal seismic velocity variation
Fernando Lawrens Hutapea, Takeshi Tsuji* and Tatsunori Ikeda
Earth, Planets and Space 2020, 72:19 DOI: 10.1186/s40623-020-1147-y
Received: 9 October 2019, Accepted: 3 February 2020, Published: 14 February 2020

Abstract
To continuously monitor crustal behavior associated with earthquakes, magmatic activities and other environmental effects (e.g., tides and rain
precipitation), we have developed a continuous monitoring system of seismic velocity of the Japanese Islands. The system includes four mainprocessing procedures to obtain spatio-temporal velocity changes: (1) preparing ambient-noise data; (2) creating virtual seismograms between
pairs of seismometer stations by applying seismic interferometry; (3) estimating temporal velocity variations from virtual seismograms bystretching interpolation approach, and (4) mapping spatio-temporal velocity variations. We developed a data-processing scheme that removes
unstable stretching interpolation results by using the median absolute deviation technique and a median filter. To map velocity changes withhigh stability and high temporal resolution during long-term (i.e., longer-termmonitoring), we proposed the “sliding reference method”. We also developed
evaluation method to select the optimum parameters related to stability andtemporal resolution. To reduce computation time for continuous monitoring,
we applied parallel computation methods, such as shared memory and hybrid
distributed memory parallelization. Using our efficient and stable monitoring
system, we succeeded to continuously monitor the spatio-temporal velocity
variation of the whole Japanese Islands using ambient-noise data from 767
seismometers. Finally, we developed a web application that displays spatiotemporal velocity changes. In the monitoring results that we open through thewebsite, we identified velocity variation (e.g., pore pressure variation) that could
be related to earthquake, aftershock, magmatic activities and environmental
effects in a stable manner.

Keywords: Crustal monitoring, Seismic velocity, Parallel andhigh-performance computing, Big data, Seismic
interferometry, Ambient noise

Graphical abstract*Corresponding author: Takeshi Tsuji, tsuji@mine.kyushu-u.ac.jp
18

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Contents
Special issue “Crustal dynamics: toward integrated view of island arc seismogenesis”....................... Toru Matsuzawa, Ichiko Shimizu, Takuya Nishimura, Christopher J. Spiers, Junichi Nakajima and Tatsuhiko Kawamoto
Frictional and structural controls of seismic super-cycles at the Japan trench............................................................Sylvain Barbot
Temporal stability of coda Q in the northeastern part of an inland high strain rate zone, central Japan: implication of apersistent ductile deformation in the crust .........................................................................Masanobu Dojo and Yoshihiro Hiramatsu
Causes of the N–S compressional aftershocks of the E–W compressional 2008 Iwate–Miyagi Nairiku earthquake (M7.2)
in the northeastern Japan arc ...............Sumire Maeda, Toru Matsuzawa, Keisuke Yoshida, Tomomi Okada and Takeyoshi Yoshida
P-wave first-motion polarity determination of waveform data in western Japan using deep learning..................................................................................................................................................Shota Hara, Yukitoshi Fukahata and Yoshihisa Iio
Hydrological changes after the 2016 Kumamoto earthquake, Japan......................................................................... Naoji Koizumi, Shinsuke Minote, Tatsuya Tanaka, Azumi Mori, Takumi Ajiki, Tsutomu Sato,
Hiroshi A. Takahashi and Norio Matsumoto

Elastic wave velocity and electrical conductivity in a brine-saturated rock and microstructure of pores................................. Tohru Watanabe, Miho Makimura, Yohei Kaiwa, Guillaume Desbois, Kenta Yoshida and Katsuyoshi Michibayashi
Fault weakening caused by smectite swelling..................................................................... Jun Kameda, Masaoki Uno, Marianne Conin, Kohtaro Ujiie, Yohei Hamada and Gaku Kimura
Temporal changes in the distinct scattered wave packets associated with earthquake swarm activity beneath theMoriyoshi-zan volcano, northeastern Japan ....................................................Yuta Amezawa, Masahiro Kosuga and Takuto Maeda
Water distribution in quartz schists of the Sanbagawa Metamorphic Belt, Japan: infrared spectroscopic mapping andcomparison of the calibrations proposed for determining water contents .......................Jun-ichi Fukuda and Ichiko Shimizu
Fully dynamic earthquake sequence simulation of a fault in a viscoelastic medium using a spectral boundary integral equation
method: does interseismic stress relaxation promote aseismic transients? ...........................................Yuki Miyake and Hiroyuki Noda
Contribution to crustal strain accumulation of minor faults: a case study across the Niigata–Kobe Tectonic Zone, Japan................................................................. Tomonori Tamura, Kiyokazu Oohashi, Makoto Otsubo, Ayumu Miyakawa and Masakazu Niwa
Simple kinetic model for replacement reactions involving solid solutions: the significant role of geofluids.................................................................................................................................................................... Tatsu Kuwatani and Mitsuhiro Toriumi
Identification of a nascent tectonic boundary in the San-in area, southwest Japan, using a 3D S-wave velocity structure
obtained by ambient noise surface wave tomography ................Yudai Suemoto, Tatsunori Ikeda, Takeshi Tsuji and Yoshihisa Iio
Detection of plastic strain using GNSS data of pre- and post-seismic deformation of the 2011 Tohoku-oki earthquake.................................................................................................................. Yukitoshi Fukahata, Angela Meneses-Gutierrez and Takeshi Sagiya(U–Th)/He thermochronometric mapping across the northeast Japan Arc: towards understanding mountain building
in an island-arc setting ..................................................................Shoma Fukuda, Shigeru Sueoka, Barry P. Kohn and Takahiro Tagami
Structural control and system-level behavior of the seismic cycle at the Nankai Trough............................................................ Qibin Shi, Sylvain Barbot, Shengji Wei, Paul Tapponnier, Takanori Matsuzawa and Bunichiro Shibazaki
Preparation zones for large crustal earthquakes consequent on fault-valve action ............................................Richard H. Sibson
Dynamic rupture simulation of 2018, Hokkaido Eastern Iburi earthquake: role of non-planar geometry...................................................................................... Tatsuya Hisakawa, Ryosuke Ando, Tomoko Elizabeth Yano and Makoto Matsubara
Deep groundwater discharge after the 2011 Mw 6.6 Iwaki earthquake, Japan..................................................................................................... Tsutomu Sato, Kohei Kazahaya, Norio Matsumoto and Masaaki Takahashi
Inelastic strain in the hypocentral region of the 2000 Western Tottori earthquake (M 7.3) inferred from aftershock
seismic moment tensors .....................................................................Satoshi Matsumoto, Yoshihisa Iio, Shinichi Sakai and Aitaro Kato
Geological structures controlled the rupture process of the 2011 M9.0 Tohoku-Oki earthquake in the Northeast Japan Arc............................................................................................................................................................................... Kei Baba and Takeyoshi Yoshida
Original composition and formation process of slab-derived deep brine from Kashio mineral spring in central Japan................................................................................................. Fumitake Kusuhara, Kohei Kazahaya, Noritoshi Morikawa, Masaya Yasuhara,
Hidemi Tanaka, Masaaki Takahashi and Yuki Tosaki

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Origin of the seismic belt in the San-in district, southwest Japan, inferred from the seismic velocity structure of the lower
crust........................................................................................................................................... Hiroo Tsuda, Yoshihisa Iio and Takuo Shibutani 16
Inelastic deformation zone in the lower crust for the San-in Shear Zone, Southwest Japan, as observed by a dense GNSS
network............................................................................................................................... Angela Meneses-Gutierrez and Takuya Nishimura 16
Inelastic behavior and mechanical strength of the shallow upper crust controlled by layer-parallel slip in the high-strainzone of the Niigata region, Japan .........................................Makoto Otsubo, Ikuo Katayama, Ayumu Miyakawa and Takeshi Sagiya 17
Stationarity of aftershock activities of the 2016 Central Tottori Prefecture earthquake revealed by dense seismic observation..................Yoshihisa Iio, Satoshi Matsumoto, Yusuke Yamashita, Shin’ichi Sakai, Kazuhide Tomisaka, Masayo Sawada, Takashi Iidaka,
Takaya Iwasaki, Megumi Kamizono, Hiroshi Katao, Aitaro Kato, Eiji Kurashimo, Yoshiko Teguri, Hiroo Tsuda and Takashi Ueno

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Coulomb stress change on inland faults during megathrust earthquake cycle in southwest Japan.............................................................................................................................................................. Tsukasa Mitogawa and Takuya Nishimura 18
Real-time crustal monitoring system of Japanese Islands based on spatio-temporal seismic velocity variation.......................................................................................................................... Fernando Lawrens Hutapea, Takeshi Tsuji and Tatsunori Ikeda 18
Cover image: Barbot, S., Earth, Planets and Space 2020, 72:63 DOI: 10.1186/s40623-020-01185-3